This invention relates to a method of coating conductors with inorganic insulation, and more particularly for coating superconducting niobium-tin (Nb.sub.3 Sn) wire, or for coating conventional conductors (for use at ambient temperatures) with a high temperature and radiation resistant inorganic insulation.
Fast pulse superconducting magnets for use in airborne AC generators, MHD generators, and energy storage devices require high current, high field, low loss, stable superconductors and associated insulation systems. The multifilamentary Nb.sub.3 Sn superconductor has emerged as the most promising conductor for use in these machines. However, while the electrical properties of multifilamentary Nb.sub.3 Sn, with regard to both high current density and low loss, are advantageous, its mechanical properties lead to other problems. A major concern in developing an insulation system for Nb.sub.3 Sn is the high formation temperature (600.degree.-800.degree. C.) of Nb.sub.3 Sn and the fact that it is a brittle intermetallic compound and after its formation cannot be readily deformed. Its formation reaction at the formation temperature must be carried out when the wire is in its final geometry. An ideal insulation system would, therefore, be one that will not only withstand the breakdown voltage of several hundred volts at liquid He temperature but also withstand the superconductor reaction temperature of 600.degree.-800.degree. C. The insulation should be capable of being applied before reaction, be able to withstand the time-temperature excursion during reaction, and have no adverse effects on the electrical and mechanical properties at low superconducting temperature. In addition, it should provide good thermal contact between the wire and the enthalpy stabilizer, which is usually liquid helium, or in the best case, provide a measure of stabilization itself. This requires that the insulation must withstand this high formation temperature and yet be electrically satisfactory at low use temperature (4.degree.-8.degree. K.). A further requirement of the insulation is that is must absorb the energy dissipated during fast charge and discharge of the magnet. The energy must be absorbed without allowing the temperature of the conductor to rise high enough to quench the magnet.
U.S. Pat. Nos. of interest include 4,407,062 to Sutcliffe et al; 4,261,097 to Weisse; 4,178,677 to Weisse; 3,985281 to Diepers; and 3,749,811 to Bogner et al. The Sutcliffe et al '062 patent relates to a method of insulating superconductive wire by coating a Nb.sub.3 Sn precursor wire with a layer of a mixture of a silicate of sodium, lithium or potassium and a second component capable of reacting with the silicate to form a ceramic, such as alumina, drying the coating and heating the coating to a temperature in excess of 500.degree. C. to react the silicate and the second component to form the insulating ceramic. The Weisse '097 patent disclosed insulating superconductor magnet windings with ceramics, glass, or quartz in the form of filaments, fabrics or nonwoven fabrics. The remaining references are of general interest.
The usual approach to electrical wire insulation for conventional conductors is to use organic enamel composed of organic high molecular materials, such as polyesters and polyimides. In many cases, these coatings are protected by a metal sheath for mechanical protection. The temperature capabilities of these organic coatings are between 100.degree.-200.degree. C., rendering the wire very susceptible to fire and toxic fumes. In addition, they suffer from radiation damage in a nuclear environment. On the other hand, as a result of rapid technological process in aerospace and nuclear power developments, electrical wire manufacturers today are faced with a small supply of super heat-resistant and radiation-resistant enameled wires which are beyond the limits of conventional wires with organic insulation. In the past, the use of inorganic coating such as ceramics has been severely limited due to difficulty in fabrication and to the brittle nature of these materials.
U.S. Pat. No. 4,429,007 to Bich et al covers an electromagnetic coil for high temperature and high radiation application in which glass is used to insulate the electrical wire. A process for applying the insulation to the wire is disclosed which results in improved insulation properties.
Other U.S. Pat. Nos. of interest include 3,883,370 to Kanter; 3,446,660 to Pendleton; 3,442,702 to Pendleton et al; 3,291,638 to Stadlen et al; 3,223,553 to Morey; 3,222,219 to Saunders et al; 3,089,787 to Sattler et al; 3,078,186 to Tierney; 3,109,053 to Ahearn; and 3,119,897 to Coper.
The Pendleton et al '702 patent disclosed a high temperature electrical conductor insulated with a fused glass matrix free of boron and is alumina-free, but does contain calcium fluoride. Refractory chromium oxide, alumina, silica or titania particles are intermixed with the glass frit in the resin and solvent of the enamel coating. The copper conductor has a nickel or nickel alloy surface. The glass particles equal 50-100% of the weight of the resin. The glass is mixed with 10-30% of its weight of the refractory oxide such as Cr.sub.2 O.sub.3 also dispersed in the resin. The Saunders et al '219 patent contains disclosures relating to separate glass frit and refractory material phases. The remaining references are provided as being of general interest.
The Encylopedia of Chemical Technology, Ed. by Kirk-Othmer, third edition, Vol. 11, at pages 807-890 has an article on glass, and at pages 881-890 has an article on glass-ceramics. Note particularly pages 826-827 for a table, hereby incorporated by reference, which includes the composition of Corning glasses 7052 and 7570.